1) they will make you buy a stamp for every 'assault' type weapon you own. You must come register every gun that fits some description and pay an increasing fee every year on said weapon.

2) when they've had enough fun registering and taxing for awhile and they have a good list of who owns the guns, they will be able to refuse to reregister the guns anymore, making the guns illegal.

They will have you by the castanets then: People who have legally paid and jumped through the hoops for awhile are unlikely to hide their now illegal rifles, while the criminals who never registered them wont care.